The manufacture of semiconductor articles, such as wafers, consists of forming various materials layers and structures of certain different thicknesses. Usually, this process includes deposition and removal of different materials using such techniques as CMP, CVD, etching, photolithography, etc. An important step in these procedures is terminating the process after the desired thickness is reached. For example, when dealing with CMP or etching, this process should be terminated after the layer being etched or polished is removed (e.g., partly removed such that a required remaining thickness of this layer is reached), or before the next, underlying layer is removed. A technique of determination of that process point at which the processing should be stopped is called “endpoint detection”.
The term “processing” used herein signifies at least one of the following: removing an uppermost layer or depositing a layer of a different material onto the wafer's surface. An endpoint detector serves to determine whether the desired thickness of the layer being removed or deposited is reached, aimed at terminating the removing or deposition process. In most cases, the process is terminated in response to a predetermined signal generated by such an end-point detector (or a plurality of such detectors).
CMP is a known process aimed at the planarization of the surface of the uppermost wafer's layer. CMP is basically a mechanical polishing of the wafer's surface using a pad pressed against the wafer, rotating one with respect to the other, all in a chemical liquid environment, which enhances the polishing Like any semiconductor process step, tight control of the CMP process is required to maintain high yield levels. The polishing removal rate, which is the main process characteristic, is a complex function of different parameters which are partly controlled or understood. These dependencies, when combined with requirements for high uniformity levels and tight process reproducibility and control, dictate intensive thickness measurement procedures, notably in oxide polishing that has no natural end-point. As a result, monitoring systems and methods are a crucial part of the CMP process.
Chemical Vapor Deposition (CVD) and etching are two other major sub-processes in the semiconductor production. The former is aimed at depositing thin films (e.g., oxides, metals) on a semiconductor wafer, whereas the latter is aimed at patterning thin films according to a developed three-dimensional image on the films. In a similar manner to CMP, both CVD and etching are influenced by various parameters, and should therefore be tightly monitored and controlled in order to achieve the set targets of the process. As for the photolithography technique, similar processes, namely, photoresist coating (e.g., by spinning) and photoresist development (i.e., selective removing by etching) take place during the photoresist processing step.
The following are three major techniques used for controlling one of the above processes of semiconductor manufacture, discussed with respect to CMP:
(1) Stand Alone (SA) Systems.
An SA system is installed outside the production line (‘off-line’) and wafers to be measured by this system are supplied thereto from the production line after the wafer processing is completed. The known SA systems for CMP are OptiProbe 2500, commercially available from ThermaWave, USA, and UV1250, commercially available from KLA-Tencor, USA. SA systems have excellent capability to provide full and accurate information concerning the measurement parameters. However, SA systems suffer from several drawbacks such as a lag in response time, large foot-printing, and clean room and additional handling of wafers issues.
(2) In-Situ Detectors
These are various sensors (optical, electrical, mechanical, etc.) which are installed in the working area (‘in-situ’) of the processing tool (e.g., the area between the wafer and the rotating pad of the polisher), are capable of real-time detecting the process end-point (e.g., motor current), and of continuously detecting the product parameters (e.g., thickness) and both product and process parameters (e.g., removal rate). Such an in-situ end-point detector (EPD) to be used with CMP equipment is disclosed, for example, in U.S. Pat. No. 5,433,651. The end-point detector comprises a window, which enables in-situ viewing of the polishing surface of the workpiece from an underside of the polishing table during polishing. Reflectance measurement means are coupled to the window on the underside of the polishing table. A prescribed change in the in-situ reflectance corresponds to a prescribed condition of the polishing process.
An EPD reduces the time required to qualify a process, and shortens conditioning time whenever pads are replaced. EPDs are mainly used in processes such as plasma etching. The known EPD tools for CMP are models 2350/2450 Endpoint Controllers, commercially available from Luxtron, Santa Clara, USA, and ISRM, commercially available from Applied Materials, Santa Clara, USA.
Unfortunately, EPD suffers from the following drawbacks: When applying the CMP to dielectric layers (which is a so-called “blind stop” process), additional frequent post-polish measurements on SA systems are needed. This is associated in the following. The EPD sensor is located in the interior of the processing area, and measures average data over a relatively large area comprising different and variable patterns. As a result, it cannot provide information concerning local planarization, and is therefore less informative as compared to an SA tool. The average data generated by the EPD does not allow for mapping the wafer's plan, whereas the latter may be of high importance. Additionally, the interpretation of in-situ sensor data is complex and less accurate, since it is also affected by irregular environment characteristics such as electrical noise, slurry, mechanical movement, etc. The in-situ EPD has low accuracy due to low optical resolution and strong signal dependency on wafer's pattern.
To demonstrate problems arising from the detection of the layer's end of polish with an in-situ EPD, reference is made to FIGS. 1 and 2. FIG. 1 illustrates a common structure, generally designated 1, of stack layers on a semiconductor wafer W, which structure is to be polished. The structure 1 contains a silicon substrate 2, a Silicon Nitrate layer (Si3N4) 4, and a top Silicon Oxide layer (SiO2) 6. FIG. 2 illustrates possible signal time changes determined by an EPD sensor during the CMP process applied to the two upper layers 4 and 6. In this example, the part A presenting a substantially “flat” graph indicative of slow signal variations corresponds to the signal detected from the upper Silicon Oxide layer 6 being polished. When the layer 6 is almost completely removed, a varying signal (part B) is detected, which changes faster with the layer's disappearance. At last, when the Silicon Nitrate layer 4 is being polished, a substantially slow changing signal is observed (part C). The signal boundaries between the parts A and B, and B and C are not sharp and clear. Hence, simple threshold-based signal analysis may cause failures, either because of “early detection” (the layer to be polished is not sufficiently removed) or because of “late detection” which means that the undesirable removal of the lower layer has started.
The main difficulty in obtaining high accuracy in optical EPD is signal dependency on wafer pattern, since EPD spot size includes a lot of features with different layers structure. The effect may be stronger than signal change during polishing There is a great variety of approaches aimed at increasing the accuracy of the endpoint detection. U.S. Pat. No. 5,910,011 discloses a method and apparatus for in-situ monitoring, using multiple process parameters. This technique utilizes analyses of the multiple process parameters and statistical correlation of these parameters to detect changes in process characteristics, such that the endpoint of the etching process may be accurately detected. Another improved endpoint technique is disclosed in U.S. Pat. No. 5,964,980. Here, a fitted endpoint system provides normalizing the current endpoint curve generated from the series of multi-bit digital code words for a wafer being etched with respect to the standard endpoint curve and providing a normalized current endpoint curve.
However, none of the known EPDs provides required measurement performance, equal or similar to SA measurement tools.
(3) Integrated Monitoring (IM) Technique
An integrated monitoring tool (IMT) is installed inside or attached to the process equipment (PE), at a location where a wafer can be monitored immediately after completion of the process, while still within the internal environment of the PE (i.e., ‘in-line’ monitoring). Wafers are supplied to the IMT by the PE's robot. IMT can be used for a CMP (e.g., integrated thickness monitoring (ITM) tool such as ITM NovaScan 210, commercially available from Nova Measuring Instruments Ltd., Israel), etching and CVD processes. The IMT combines the performance of a SA tool with short time-to-respond of usually one wafer delay only, i.e., not much longer than the real-time response of an EPD. Consequently, an IMT has advantages over SA tool and provides additional important information, as compared to the EPD system, with practically no performance loss. These advantages are emphasized with respect to the ITM apparatus:
The ITM measurement unit provides thickness measurement data for every product wafer, hence, enabling fast feed-back or feed-forward control of the CMP. Measurements are carried out in parallel to processing the next wafer(s), thus, there is no affect on PE throughput.
Some known techniques utilizing the principles of ITM for closed-loop control are disclosed in the following articles: “Dielectric CMP Advanced Process Control Based on Integrated Thickness Monitoring”, VMIC Specialty Conference, Santa Clara, 1997; and “Oxide Chemical mechanical Polishing Closed Loop Time Control”, CPIE, Vol. 3882, Santa Clara, 1999.
Although such problems as the wafer handling, clean room space and labor needed for SA tools operations are completely eliminated in the ITM, the latter still does not give a real-time response, but rather a post-factum measurement of the CMP process, and cannot eliminate the problem of different thicknesses of the processed layer that might happen during processing of at least one wafer.
U.S. Pat. Nos. 5,658,183 and 5,730,642 disclose a specific system for polishing a semiconductor wafer, wherein the ITM tool (NovaScan 210) and an in-situ detector are used. The in-situ detector is aimed at controlling various process parameters, while the end-point detection aimed at determining whether the polishing of the wafer is complete is performed by interrupting the polishing process and performing repetitive measurements with the ITM tool. It is evident that this technique does not provide real-time endpoint detection.